1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly relates to a semiconductor light emitting device which is provided with concaves/convexes on a surface for a light extraction (hereinafter, referred to as light extraction surface) for increasing light extraction efficiency.
2. Description of the Related Art
Conventionally, a compound semiconductor, for example, a gallium nitride (GaN) has been used as a common material for a semiconductor light emitting device such as an LED. In the semiconductor light emitting device described above which uses a GaN-based compound semiconductor, a plurality of concaves/convexes may be formed on a semiconductor layer surface (see paragraph [0050] and FIG. 1 in JP 2000-196152, and paragraphs [0054], [0088], FIG. 1 and FIG. 20 in JP 2007-88277) or a substrate (see paragraph [0042] and FIG. 2 in JP 2003-69075, paragraph [0045] and FIG. 3(b) in JP 2007-67209, and paragraphs [0054], [0088], FIG. 1 and FIG. 20 in JP 2007-88277) for increasing light extraction efficiency of a semiconductor layer which is stacked including a GaN-based light emitting layer in most cases.
In the light emitting device disclosed in JP 2000-196152, many hemispherical convexes are formed spaced each other on a light extraction surface of a GaN-based semiconductor layer which is grown on a sapphire substrate. This is because if the light extraction surface is a flat surface, an obliquely incident light on the flat surface is totally reflected. In the light emitting device, the convex enables the obliquely incident light on the light extraction surface (convex) to pass outside, depending on an angle between a surface of the convex and the incident light.
In the light emitting device disclosed in JP 2003-69075, a substrate is composed of a GaN-based compound semiconductor. In the GaN-based compound semiconductor substrate, a pit (hole, concave) is formed on a surface opposite to a surface on which a device made of, for example, GaN-based semiconductor layer is formed. In the pit, for example, a plurality of planes appear in a step-like pattern and a slope is formed between the planes.
In the light emitting device disclosed in JP 2007-67209, a undoped GaN substrate which has a dislocation density not more than 106/cm2 is used. In the undoped GaN substrate, macro concaves/convexes (step: 3 μm) are formed by grinding using a grinder on a surface opposite to a surface on which a device made of, for example, GaN-based semiconductor layer is formed, and the ground surface is further chemically treated (dry etching) to form circular cones in high density as micro concaves/convexes (step: not more than 1 μm). In the JP 2007-67209, the final purpose is to realize a high light emitting efficiency, while targeting a reduction of a driving voltage of the light emitting device as a firsthand purpose by disposing the micro convexes in high density. Hence, the light emitting device is provided with the macro convexes in a pre-treatment for disposing the micro convexes in high density finally.
The light emitting device disclosed in JP 2007-88277 is a face-down type device (flip chip). Two types of convexes are formed using a pattern either on a surface opposite to a surface on which a GaN-based semiconductor layer is formed on a sapphire substrate or on a surface of n-type semiconductor layer after removal of the sapphire substrate subsequent to a formation of each electrode. One is a first convex (1 μm) which is formed in a long-period and relatively high. The other is a second convex (0.3 μm) which is formed in a short-period and relatively low. In the technology disclosed in JP 2007-88277, an interval between convexes is set such that, for example, the long-period is not less than 2.3 μm and the short-period is not less than 0.46 μm, based on the experimental data to increase the light extraction efficiency.
However, when concaves/convexes are disposed for increasing the light extraction efficiency, there are various problems depending on a structure of the concaves/convexes. For example, in the light emitting device disclosed in JP 2000-196152, since the convex formed on a semiconductor layer surface is formed in a hemispherical shape, the light extraction efficiency of a light emitted outside from the convex in the right upward direction becomes low, thereby resulting in poor light distribution.
If the structure is a concave, it is expected that light extraction efficiency may be increased by making the concave deeper. For example, when an opening area (surface side portion) of the concave is fixed and the concave has a taper shape in a depth direction from the opening, and if a surface (side surface) of the concave in the depth direction is steeply inclined, a deeper concave may be formed. However, when an electrode (n-electrode or p-electrode) is formed on a semiconductor layer surface where a plurality of concaves/convexes are disposed as the conventional technology, if a deep concave is formed, a current diffusion from the electrode is likely to become poor due to the deep concave even if the electrode is not disposed right above the concave. As a result, there is a tendency that a current flowing downward from the electrode becomes dominant in the total current on the horizontal plane. In addition, to uniformly increase the current diffusion in the horizontal direction from the electrode, it is necessary to increase a film thickness of the semiconductor layer, where the concave is disposed, right beneath the electrode in comparison with a semiconductor layer where the concave is not disposed.
When the electrode is further stacked as described above, it is necessary to form a depth of the concave relatively shallow by making an inclination of the side surface of the concave gentle. It may be possible to form an opening portion of the concave larger in accordance with a shortened depth of the concave by making the inclination of the side surface of the concave gentle. In this case, however, the concaves/convexes become relatively small, and as a result, the light distribution becomes poor due to approaching to a flat surface.
The pit as a concave of the light emitting device disclosed in JP 2003-69075, on which an electrode is stacked, is disposed for suppressing an interference to be generated by multiple reflection of light inside the semiconductor device. Hence, in JP 2003-69075, there is no description on what structure of the pit improves the light extraction efficiency.
In the light emitting device disclosed in JP 2007-67209, a convex is formed on a backside of an undoped GaN substrate for achieving a reduction of a driving voltage, which is a firsthand purpose. Namely, the convex is not disposed on a light extraction surface of a GaN-based semiconductor layer surface. Therefore, there is no direct relation between a convex structure and light extraction efficiency of a surface on the light extraction side of a semiconductor layer including a light emitting layer.
With respect to the light emitting device disclosed in JP 2007-88277, it is required that positions of a first convex and a second convex are accurately arranged based on a unique theory and experimental data for improving the light extraction efficiency. In addition, a light emitting device other than a facedown type (flip chip) can not be applied to the light emitting device disclosed in JP 2007-88277.
The present invention has been developed considering the foregoing problems, and it is an object of the present invention to provide a semiconductor light emitting device which has high light extraction efficiency of a surface on a light extraction side of a semiconductor layer including a light emitting layer and a good light distribution.